I'm also curious about this. Couldn't yubico release the U2F applet and have it just have a self-signed attestation certificate?
Or is the problem that there wouldn't be any USB support?
I guess it really depends on the block diagram for the yubikey neo. My guess is that there are two micro controllers: The USB front-end micro controller and the secure element. The code running on the secure element is, in theory, upgradable, however the firmware on the USB front-end micro controller would not be upgradable. You'd need to have explicit U2F support in the USB front-end micro controller in order to allow your yubikey to be upgraded to USB U2F support. This is, of course, based on assumptions of how the Yubikey NEO hardware is implemented.
(Curiously, if this is the case, NFC U2F support, however, when finalized, wouldn't be out of the question)
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